We bought a hardware router, which is meant to replace the linux machine, which has the IP 192.168.0.250.

Now I want to keep the linux machine running and routing, while it uses the new hardware router. I need this, so I don't have to change all the configurations of the clients at once (no DHCP here).

Question: do I have to use two ethernet cards, like in the current state, where eth0 is connected to a DSL modem?
Or can I just happily connect eth1 to the network of the new hardware router and tell the linux machine that it forwardes everything to 192.168.250?